Stop Collar

ABSTRACT

Stop collars may be fixedly mounted on a liner or other tubular member. The stop collar comprises a first sleeve and a second sleeve. The sleeves are adapted to extend around the outer circumference of the tubular member and are engaged by threaded connections. The sleeves provide opposing inner thrust surfaces. The collar further comprises a radially expandable gripping member disposed between the opposing thrust surfaces. The sleeves may be drawn together by rotation about the threaded connection to cause the thrust surfaces to bear on the gripping member and expand the gripping member radially inward into frictional engagement with the tubular member.

FIELD OF THE INVENTION

The present invention relates to downhole tools used in oil and gas welldrilling operations and, and more particularly, to tool assemblies andstop collars that may be mounted on a liner or other tubular used in oiland gas wells.

BACKGROUND OF THE INVENTION

Hydrocarbons, such as oil and gas, may be recovered from various typesof subsurface geological formations. The formations typically consist ofa porous layer, such as limestone and sands, overlaid by a nonporouslayer. Hydrocarbons cannot rise through the nonporous layer. Thus, theporous layer forms a reservoir, that is, a volume in which hydrocarbonsaccumulate. A well is drilled through the earth until the hydrocarbonbearing formation is reached. Hydrocarbons then are able to flow fromthe porous formation into the well.

In what is perhaps the most basic form of rotary drilling methods, adrill bit is attached to a series of pipe sections or “joints” referredto as a drill string. The drill string is suspended from a derrick androtated by a motor in the derrick. A drilling fluid or “mud” is pumpeddown the drill string, through the bit, and into the bore of the well.This fluid serves to lubricate the bit. The drilling mud also carriescuttings from the drilling process back to the surface as it travels upthe wellbore. As the drilling progresses downward, the drill string isextended by adding more joints of pipe.

A modern oil well typically includes a number of tubes extending whollyor partially within other tubes. That is, a well is first drilled to acertain depth. Large diameter pipes, or casings, are placed in the welland cemented in place to prevent the sides of the borehole from cavingin. After the initial section has been drilled, cased, and cemented,drilling will proceed with a somewhat smaller wellbore. The smaller boreis lined with somewhat smaller pipes or “liners.” The liner is suspendedfrom the original or “host” casing by an anchor or “hanger.” A well mayinclude a series of smaller liners, and may extend for many thousands offeet, commonly up to and over 25,000 feet.

The liners are cemented in the wellbore as the well is constructed. Thatis, the liner necessarily is smaller than the bore in which it isinstalled. The gap between the liner and the wellbore is referred to asthe annulus, and it is filled with cement after the liner has beeninstalled. The cement helps secure the liner in the wellbore and protectit against corrosion and erosion. It also supports the borehole wallsfrom collapse. If fluids will be produced through the liner, cement alsohelps ensure more precise control over stimulation processes, such asfracturing and acidizing.

Most importantly, the cement is intended to form a continuous sheath,that is, a complete seal around the liner. If the liner leaks, thecement sheath will help ensure that fluids flowing through the liner donot contaminate the surrounding formation, and especially water-bearingformations. The cement sheath also ensures that hydrocarbons and otherfluids in the formation are not able to migrate to other parts of theformation or to the surface.

The liner is cemented in the wellbore by injecting a cementitious,settable slurry down the liner and allowing it to flow up the annulus.The cement then is allowed to set, that is, solidify and harden intowhat hopefully will be a continuous seal throughout the annulus. Thereare a number of challenges, however, in ensuring that the sheath iscontinuous and that a complete seal is established between the bore andliner. Some issues arise from the chemical and physical nature of thecement slurry and how it interacts with other fluids in the well or theformation. The position of a liner in the bore also can createsignificant impediments to forming a complete seal.

That is, oil wells are commonly depicted as extending straight down intothe earth with a tube running right down the middle of the bore. Thetruth is far from that. Because it is formed with a rotating drill bit,the bore will tend to corkscrew as it is extended. Moreover, in order toincrease production, the bore commonly will be deviated from a nominalvertical bearing to extend it along, rather than through ahydrocarbon-beating formation. So-called “horizontal” wells constitutemost of the wells being drilled in the United States today.

A liner, therefore, will not necessarily be centered within a wellbore.It may tend to rest against the side of a bore, especially in horizontalextensions. A cement slurry may not be able to flow into the area wherea liner rests against the borehole. Thus, when set, voids may be left inthe cement sheath, or it may have thin, weak portions. Fluids from theformation may be able to migrate from one area of the formation toanother or may even reach the surface.

In an effort to mitigate such problems, a liner typically will beprovided with centralizers. The centralizers are intended to maintain aminimum clearance between the liner and the bore, while at the same timeproviding paths which allow cement slurry to flow past them.“Bow-spring” centralizers are one common type of centralizer. They havea pair of relatively short sleeves that fit around the outside of aliner. A number of relatively stiff, narrow bow springs extend betweenthe sleeves. The bow springs curve away from the liner and keep theliner spaced from the walls of the bore. The springs are spacedangularly around the circumference of the sleeves so that cement slurrycan flow around and through the centralizer.

“Spiral-blade” centralizers are another common type. They incorporate asingle longer sleeve. The sleeve has raised, rounded blades that extendalong the sleeve in a loose helix. The blunt blades maintain clearancewith the bore while providing channels through which cement may flow.Centralizers, therefore, can greatly reduce or eliminate contact betweena liner and the bore, and help ensure that the cement sheath will havesufficient thickness throughout the annulus to provide an effectiveseal.

Centralizers may be mounted on a liner in a fixed position. For example,U.S. Pat. Pub. No. 2013/0160993 of J. Davilla et al. discloses aspiral-blade centralizer that is fixedly mounted on a liner. Itgenerally comprises a body and a pair of wedge rings. The body isgenerally cylindrical and fits around a liner. Blunt blades are providedaround its circumference to keep the liner spaced from the bore andprovide channels for cement flow. Each end of the centralizer body hasinternal threads. The threads are opposed. That is, the threads on oneend of the body are right-hander and those at the other end areleft-handed. The internal threads on the body engage external threads ona tapered surface of the wedge rings. Thus, the body may be rotated todraw the wedge rings together. As they draw together, teeth on the innersurface of the wedge rings bite into the liner, securing the centralizerin place.

Providing a centralizer with some freedom of movement, however, can makeit much easier to run a liner into a well. Thus, other centralizers,including bow-spring and spiral-blade type centralizers, are mountedsuch that they are free to rotate and travel a certain distance along aliner. Sliding centralizers are widely available on the market,including slip-on bow-spring and spiral-blade centralizers distributedby Top-Co, Houston, Texas, and MSIS-B bow spring centralizers availablefrom Weatherford. Movement of the centralizer along a liner will belimited by what are referred to as thrust or stop collars. A stop collarwill be placed above and below the centralizer as a joint of liner isrun into the well. The stop collars are securely mounted to the liner toprovide mechanical stops limiting travel of the centralizer along theliner.

Stop collars may simply comprise a collar which is slid on a liner andsecured in place with set screws. Other designs utilize a hinged collaror a split collar. The collar is opened to place it around a liner. Theends then are brought together and latched or otherwise secured. Suchdesigns may have metal gripping features which bite into the liner, orthey may have an elastomeric layer concentrically disposed within thecollar. Some designs incorporate both metal gripping features andelastomers, such as the stop collars disclosed in U.S. Pat. No.3,652,138 to C. Collett. Other designs rely on a layer of swellableelastomer disposed on the inside surface of the collar, such as thosedisclosed in U.S. Pat. No. 7,942,199 to P. Angman. The collar is dippedinto an activating solution and then slipped onto the liner. Theelastomer swells and grips the liner.

The statements in this section are intended to provide backgroundinformation related to the invention disclosed and claimed herein. Suchinformation may or may not constitute prior art. It will be appreciatedfrom the foregoing, however, that there remains a need for new andimproved systems and apparatus for mounting stop collars and other toolassemblies in a fixed position on liners and other tubular members. Suchdisadvantages and others inherent in the prior art are addressed byvarious aspects and embodiments of the subject invention.

SUMMARY OF THE INVENTION

The subject invention, in its various aspects and embodiments, relatesgenerally to assemblies and stop collars that may be mounted in a fixedposition on tubular members, such as liners, that are used in oil andgas wells. It encompasses various embodiments and aspects, some of whichare specifically described and illustrated herein.

One broad embodiment of the subject invention provides a collar formounting on a tubular member. The collar comprises a first member and asecond member. The two members are adapted for lateral displacementrelative to each other and provide opposing inner thrust surfaces. Thecollar also comprises a radially expandable gripping member which isdisposed between the opposing thrust surfaces. The members may be drawnlaterally together to cause the thrust surfaces to bear on the grippingmember. As the thrust surfaces bear on the gripping member that willexpand it radially inward into frictional engagement with the tubularmember.

Other broad embodiments and aspects are directed to a collar that ismountable on a tubular member. The collar comprises a first sleeve and asecond sleeve. The two sleeves are adapted to extend around the outercircumference of the tubular member. The sleeves are engaged with eachother by threaded connections and provide opposing inner thrustsurfaces. The collar also comprises a radially expandable grippingmember which is disposed between the opposing thrust surfaces. Thesleeves may be drawn together by rotating them relative to each otherabout the threaded connection. The relative rotation will cause thethrust surfaces to bear on the gripping member and expand the grippingmember radially inward into frictional engagement with the tubularmember.

In other aspects, the invention provides for such collars where thegripping member engages the tubular member without penetrating themember, where it engages the tubular member without creatingelectrolytic contact between the collar and tubular member, or where itengages the tubular member without penetration and without creatingelectrolytic contact.

Other aspects are directed to such collars where the first sleeve isprovided with external threads engaging internal threads on the secondsleeve and the gripping member is disposed between the inner end of thefirst sleeve and an annular shoulder in the second sleeve. Still otherembodiments are directed to such collars where the second sleeveprovides radial support for the gripping member. Yet other embodimentsprovide such collars where the gripping member is carried in an expandeddiameter portion of the second sleeve

Additional embodiments and aspects provide such collars where thegripping member is a compressible band, preferably an elastomer band.

In still other embodiments, the invention provides such collars having amechanism to restrict loosening of the threaded engagement between thesleeves. In some embodiments, the mechanism comprises a pawl on onesleeve and detents angularly spaced along the inner end of the othersleeve. The pawl selectively engaging the detents to restrict looseningof the threaded engagement between the sleeves. In other embodiments,the mechanism is a set screw extending through one sleeve andfrictionally engaging the other sleeve.

The subject invention also is directed to stop collars and other toolsfor mounting on a tubular. Broad embodiments of the novel tools comprisea novel collar. Other broad embodiments are directed to a centralizingassembly. The assembly comprises a centralizer tool. The centralizertool comprises spacing members which extend radially outward beyond thetubular member and define lateral channels. The assembly furthercomprises a novel stop collar. Still other broad embodiments aredirected to tubular assemblies. The tubular assemblies comprise a novelcentralizing assembly or other novel tool.

The subject invention also provides novel methods for conducting welloperations. Such methods include methods for centralizing liners orother tubulars in a well. The methods comprise installing a novelcentralizing assembly on the tubular and running the tubular into thewell.

Finally, still other aspects and embodiments of the invention will havevarious combinations of such features as will be apparent to workers inthe art.

Thus, the present invention in its various aspects and embodimentscomprises a combination of features and characteristics that aredirected to overcoming various shortcomings of the prior art. Thevarious features and characteristics described above, as well as otherfeatures and characteristics, will be readily apparent to those skilledin the art upon reading the following detailed description of thepreferred embodiments and by reference to the appended drawings.

Since the description and drawings that follow are directed toparticular embodiments, however, they shall not be understood aslimiting the scope of the invention. They are included to provide abetter understanding of the invention and the way it may be practiced.The subject invention encompasses other embodiments consistent with theclaims set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a liner assembly 10 beingcemented in a bore 4 of a well 1, which liner assembly 10 includes firstpreferred embodiments 20 of centralizing assemblies of the subjectinvention.

FIG. 1B is a schematic illustration showing liner assembly 10 after ithas been cemented in bore 4.

FIG. 2 is an isometric view of a centralizing assembly 20 shownschematically in FIG. 1, which centralizing assembly 20 is assembled ona portion of liner 10 and includes a pair of first embodiments 30 of thestop collars of the subject invention.

FIG. 3 is an isometric view of stop collar 30.

FIG. 4 is an exploded, isometric view of stop collar 30 showing a malesleeve 34, a compressible band 33, and a female sleeve 35 from whichstop collar 30 is assembled.

FIG. 5 is an axial cross-sectional view of stop collar 30 in its set orinstalled state.

In the drawings and description that follows, like parts are identifiedby the same reference numerals. The drawing figures are not necessarilyto scale. Certain features of the embodiments may be shown exaggeratedin scale or in somewhat schematic form and some details of conventionaldesign and construction may not be shown in the interest of clarity andconciseness.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The subject invention relates generally to tool assemblies and stopcollars that may be mounted on a liner or other tubular member in afixed position and encompasses various embodiments and aspects. Some ofthose embodiments are described is some detail herein. For the sake ofconciseness, however, all features of an actual implementation may notbe described or illustrated. In developing any actual implementation, asin any engineering or design project, numerous implementation-specificdecisions must be made to achieve a developers' specific goals.Decisions usually will be made consistent within system-related andbusiness-related constraints, and specific goals may vary from oneimplementation to another. Development efforts might be complex and timeconsuming and may involve many aspects of design, fabrication, andmanufacture. Nevertheless, it should be appreciated that suchdevelopment projects would be a routine effort for those of ordinaryskill having the benefit of this disclosure.

Overview of Cementing Operations

The novel centralizing assemblies may be used, for example, to helpimprove the quality of a cement sheath that will be formed around aliner. Thus, a first preferred centralizing assembly 20 incorporating afirst preferred stop collar 30 will be described by reference to FIG. 1.FIG. 1 are schematic depictions of a liner assembly 10 being cemented inan oil and gas well 1. Liner assembly 10 incorporates a number ofcentralizing assemblies 20.

Referring first to FIG. 1A, well 1 is serviced by a derrick 2 andvarious surface and downhole equipment for pumping cement andcirculating fluids (not shown). The upper portion of well 1 is providedwith a casing 3, while the lower portion is an open bore 4 extendinggenerally horizontally through a hydrocarbon bearing formation 5.

Liner assembly 10 has been suspended from casing 3 by a liner hanger 11and extends through open bore 4. Liner assembly 10 includes varioustools, including a toe valve 12 and a float assembly 13. Float assembly13 typically includes various tools that assist in running liner 10 intowell 1 and cementing it in bore 4, such as a landing collar 14, a floatcollar 15, and a float shoe 16. Centralizing assemblies 20 are mountedaround liner 10 and serve to maintain clearance between liner 10 andbore 4.

FIG. 1A depicts well 1 as liner 10 is being cemented in bore 4. Aquantity or “pad” of cement 6 is being pumped into liner 10, out itslower end, and into the annulus between liner 10 and bore 4. As cement 6is pumped, it displaces drilling fluids 7 already present in liner 10and the annulus. Cement 6 and fluids 7 are able to flow through theannulus and past centralizing assemblies 20.

A wiper plug 17 is being pumped behind cement 6. It follows the pad ofcement 6 as it flows through liner 10. Wiper plug 13 will help clean andremove cement 6 from the inside of liner 10. It will pass through toevalve 12 and eventually seat on landing collar 14 in float assembly 13.Pumping will continue until cement 6 completely fills the annulusbetween liner 10 and bore 4.

FIG. 1B shows well 1 after completion of the cementing operation. Cement6 has been allowed to set. Centralizing assemblies 20 have enabledcement 6 to form a sheath completely around liner 10. Derrick 2 and thecementing equipment have been replaced by well head 8 and other surfaceequipment (not shown). The equipment may be used, for example, toperforate liner 10 and to penetrate cement sheath 6 in selected areas toallow hydrocarbons to flow from formation 5 into liner 10.

The terms “upper” and “lower” and “uphole” and “downhole” as used hereinto describe location or orientation are relative to the well and to thetool as run into and installed in the well. Thus, “upper” and “uphole”refers to a location or orientation toward the upper or surface end ofthe well. “Lower” or “downhole” is relative to the lower end or bottomof the well. It also will be appreciated that the course of the wellboremay not necessarily be as depicted schematically in FIG. 1. Depending onthe location and orientation of the hydrocarbon bearing formation to beaccessed, the course of the wellbore may be more or less deviated in anynumber of directions. “Axial,” “radial,” “angularly,” and forms thereofreference the central axis of the tools. For example, axial movement orposition refers to movement or position generally along or parallel tothe central axis. “Lateral” movement and the like also generally refersto up and down movement or positions up and down the tool. “Radial” willrefer to positions or movement toward or away from the central axis.

Overview of Preferred Centralizing Assemblies

As noted above, the novel centralizing assemblies may be used to ensurethat there is sufficient clearance between a liner and a wellbore sothat a continuous sheath of cement may be formed. Broad embodimentsincorporate a centralizer and one or more novel stop collars. Forexample, centralizing assembly 20 is shown in greater detail in FIG. 2.As illustrated therein, centralizing assembly 20 comprises a centralizer21 which is mounted between a pair of first preferred embodiments 30 ofthe stop collars of the subject invention.

Centralizer 21 is a spiral-blade type centralizer. It has a generallycylindrical body 22 that may be mounted on a portion of liner 10, forexample, by sliding it over a joint of liner 10 as it is made up and runinto well 1. The outer surface of body 22 is provided with raisedfeatures to space liner 10 from bore 4, such as four rounded, ratherblunt blades 23. Blades 23 extend laterally between the two ends of body22, generally along the line of a loose helix. The height of blades 23is greatest in their mid-region and tapers toward both ends of body 22.Blades 23 serve to maintain clearance between liner 10 and bore 4 whileat the same time allowing cement 6 to flow laterally past and aroundbody 22.

Centralizer 21 is not fixedly carried on liner 10. It is free to rotateabout liner 10 and to move laterally along liner 10. A first stop collar30 a is mounted above centralizer 21, and a second stop collar 30 b ismounted below centralizer 21. Stop collars 30 are fixedly mounted toliner 10. Lateral movement of centralizer 21 along liner 10, therefore,is limited by stop collars 30. Allowing such freedom of movement helpsminimize the risk that centralizer 21 will hang up in casing 3 orwellbore 4 as liner 10 is run into well 1. It will be appreciated thatfor some applications only a single stop collar 30 may be required torestrict lateral movement of centralizer 21. For example, centralizer 21may be mounted on liner 10 between a single stop collar 30 and athreaded box or other enlarged portion of a liner joint.

It will be appreciated that the novel centralizing assemblies mayincorporate any conventional centralizer requiring the use of stopcollars. Such centralizers include not only spiral blade centralizers,such as centralizer 21, but also slip-on bow spring centralizers. Manydifferent centralizers are available from Top-Co, Weatherford, and othermanufacturers and suppliers.

Overview of First Preferred Stop Collar

As noted above, the novel stop collars may be mounted on a liner orother tubular member and used, for example, to limit the lateralmovement of a tool such as a centralizer. Broad embodiments of the novelstop collars comprise first and second members which may be displacedlaterally relative to each other. A radially expandable gripping memberis disposed between opposing thrust surfaces on the displaceablemembers. The members may be drawn together to expand the gripping memberradially inward into frictional engagement with a liner.

Stop collar 30, for example, is incorporated into centralizing assembly20 to limit the lateral movement of centralizer 21. Stop collar 30 isshown in greater detail in FIGS. 3-5. As shown therein, stop collar 30generally comprises a male sleeve 34, a compressible band 33, and afemale sleeve 35. Compressible band 33 is carried within stop collar 30between male sleeve 34 and female sleeve 35. Stop collar 30 may be slidover liner 10 (not shown in FIGS. 3-5). Male sleeve 34 and female sleeve35 may be threaded together to compress band 33 and expand it radiallyinward into frictional engagement with liner 10.

More specifically, male sleeve 34 has a generally cylindrical body 41.The inner diameter of body 41 is substantially uniform and is slightlylarger than the outer diameter of liner 10. Male sleeve 34 will fitclosely, but still loose enough that it can slide easily along liner 10.The outer end 42 of male body 41 has a radial thickness and provides anannular surface normal to the circumference of liner 10. As will beappreciated from FIG. 2, stop collar 30 preferably will be mounted onliner 10 such that outer end 42 of male body 41 provides a stop surfacelimiting travel of centralizer 21 along liner 10. The inner end of body41 has a reduced outer diameter and terminates in an annular thrustsurface 43 extending radially and normal to the outer circumference ofliner 10. A substantial portion of the reduced diameter portion of theinner end of body 41 is provided with external threads 44.

Female sleeve 35 also has a generally cylindrical body 51. The innerdiameter of body 51 is sized to allow female sleeve 35, like male sleeve34, to slide easily along liner 10. The outer end 52 of female body 51preferably is chamfered to provide stop collar 30 with a tapered leadingedge to reduce the risk of centralizing assembly 20 hanging up as liner10 is run into or out of well 1. The inner end of body 51 has an area ofincreased inner diameter forming an internal, inward-facing annularshoulder or thrust surface 53. The inner end of body 51 is provided withexternal threads 54.

Compressible band 33 is an annular body which, in its uncompressed,as-assembled state, is sized to fit loosely around liner 10. Band 33 iscarried radially within the enlarged inner diameter portion of body 51of female sleeve 35. It is disposed axially between thrust surface 43 inmale sleeve 34 and thrust surface 53 in female sleeve 35. Male sleeve 34is assembled to female sleeve 35 by the engagement of their respectivethreads 44 and 54. Thus, when male sleeve 34 and female sleeve 35 arerelatively loosely engaged and compressible band 33 is relativelyuncompressed, stop collar 30 may be slid over liner 10 for installation.

Once it is slid around liner 10, stop collar 30 may be fixedly mountedto liner 10 by rotating the threaded connection to draw male sleeve 34and female sleeve 35 together. As male sleeve 34 and female sleeve 35are drawn together, they will begin to compress compressible hand 33.Band 33 is radially supported along its width by the enlarged diameterportion of female sleeve 35. Thus, as compressible band 33 is compressedbetween thrust surfaces 43 and 53, it will tend to expand radiallyinward and into engagement with liner 10, gripping it and fixedlysecuring stop collar 30 on liner 10.

The gripping member preferably will establish non-corrosive contactbetween the stop collar and liner. The contact may be non-corrosive inthe sense that it is non-penetrating. That is, the gripping member doesnot significantly abrade or score the liner. It may be non-corrosive inthe sense that it does not create electrolytic contact between the stopcollar and liner. That is, the gripping member will not createsignificant electrolysis between the stop collar and liner. Preferably,the contact will be both non-penetrating and non-electrolytic.

Compressible band 33, for example, may be made from any material capableof radially expanding into a state of elastic compression. Preferably,however, band 33 is fabricated from elastomers, such as nitrilebutadiene rubber (NBR), hydrogenated nitrile butadiene nitrile rubber(HNBR), and fluoroelastomers such as Viton®, or polytetrafluoroethylene.Such materials may allow stop collar 30 to be securely mounted withoutdamaging the surface of liner 10 and potentially shortening its servicelife.

That is, it will be appreciated that conventional collars may employ setscrews, or they may have metal teeth or other gripping features thatbite into liner 10. Liner 10 typically will be made of very hard,durable steel, but nevertheless, it will be exposed to harsh fluids,potentially from within and without, and potentially over a long servicelife. Abrasion and scoring of the metal may make liner 10 moresusceptible to corrosion and, if there is contact between dissimilarmetals, to electrolytic corrosion. When compressible band 33 isfabricated from elastomers, such issues may be avoided. Other materials,however, may be used to establish non-penetrating or non-electrolyticcontact between the stop collar and liner.

In some applications, it may be acceptable to use soft, ductile metalssuch as brass and aluminum alloys to fabricate gripping members. Suchmaterials generally are not preferred. While they typically will benon-penetrating and may not significantly score or otherwise damage aliner during installation, they still create issues of electrolyticcorrosion. Thus, as a general matter, gripping members, like anelastomer band, which establish non-electrolytic contact between theliner and the stop collar as well as non-penetrating contact arepreferred.

An elastomeric band 33 also may provide stop collar 30 with a“self-energizing” grip on liner 10. That is, as centralizer 21 bears onbottom end 42 of male sleeve 34 it will tend to urge stop collar 30 totravel along liner 10. Male sleeve 34 and female sleeve 35 fit looselyaround liner 10. They may be displaced fairly easily, especially ifsignificant torque is not created through stop collar 30. Elastomericband 33, however, is frictionally engaged with liner 10 and will resistdisplacement. Thrust surface 54 on female sleeve 35, therefore, may pullaway from band 33. Thrust surface 44, however, will bear on band 33,compressing it, further expanding it radially inward, and increasing itscoefficient of friction. The load on stop collar 30, of course, maybecome so high that it will force elastomer band 33 to skip or extrude.Until it reaches that point, however, the grip on liner 10 will increasewith load. Such dynamics also can provide a shock absorbing effect, andhelp minimize the risk of damage to centralizer 21 and stop collar 30 asliner 10 travels through the well.

A band, such as band 33, also may be preferred as it provides arelatively large area of contact around liner 10 and correspondinglymore frictional resistance to movement. Band 33 also may be providedwith various features, such as raised nubs, ridges, or other embossmentsto facilitate a frictional engagement between band 33 and liner 10. Thegripping member, however, may have various other configurations. Forexample, a plurality of discrete gripping members also may be providedand arrayed angularly about the inner circumference of the stop collars.

Similarly, the threaded connection between male sleeve 34 and femalesleeve 35 provide a simple, effective mechanism for drawing their thrustsurfaces 43 and 53 together to compress and radially expand band 33radially inward. Other mechanisms, however, may be used. For example,the stop collars may incorporate a pair of terminal or end sleeves, eachhaving a threaded end. The threads on the end sleeves may spiral inopposite directions—the threads on one sleeve being right-handed, andthe threads on the other being left-handed. A central or connectionsleeve may be provided with opposing internal threads on each end. Theconnection sleeve may be used to connect and draw the two end sleevestogether. Other conventional means of drawing opposing thrust surfacestogether may be used.

It also will be appreciated that the novel collars may be provided withother features that may make installation easier or enhance theirreliability. For example, holes 45 and 55 may be provided, respectively,on male sleeve 34 and female sleeve 35. Pins on a spanner tool may beinserted into holes 45 and 55 to rotate sleeves 34 and 35 duringinstallation or to remove them from liner 10. The novel stop collarsalso preferably will have a mechanism to ensure that once they areinstalled on liner 10, the threaded connection between male sleeve 34and female sleeve 35 will not loosen to the point where stop collar 30will slide along liner 10.

For example, as best seen in FIGS. 3-4, female sleeve 35 is providedwith a series of notches or detents 56 arrayed angularly around itsinner end. Male sleeve 34 is provided with one or more pivoting pawls 46mounted in its mid-region. Pawl 46 is spring-load to bias it intodetents 56. Detents 56 are shaped such that, when male sleeve 34 andfemale sleeve 35 are rotated to draw them together, pawl 46 will pivotinto and then ramp out of detents 56. Instead of a ramp, the other faceof detents 56 is designed to fit and catch pawl 46. Thus, once malesleeve 34 and female sleeve 35 have been drawn together fully to mountstop collar 30 on liner 10, pawl 46 will catch in a decent 56 andprevent significant separation of male sleeve 34 and female sleeve 35 orrelaxation of band 33.

It will be appreciated, of course, that suitable pawl-decent mechanismsmay have other designs and effectively prevent loosening of stop collar30. Likewise, other mechanisms for preventing relative rotation betweenmale sleeve 34 and female sleeve 35 once they have been drawn togethermay be used. For example, a set screw may be threaded through femalesleeve 35 and driven into frictional engagement with male sleeve 34.Other designs are known in the art and may be used.

In general, the novel stop collars may be fabricated from materialstypically used in stop collars of this type. Given the extreme stressand the corrosive and abrasive fluids to which they may be exposed,suitable materials will be hard and strong. For example, excepting theirgripping member as discussed above, the components of novel stop collarsmay be fabricated from 4130 and 4140 chromoly steel or from somewhatharder, stronger steel such as 4130M7, high end nickel alloys, andstainless steel. The components may be made by any number ofconventional techniques, but typically and in large part will be made byforging, extruding, or mold casting a blank part and then machining therequired features into the part.

The novel stop collars have been described as being mounted on a linerand, more specifically, a production liner. A “liner,” however, can havea fairly specific meaning within the industry, as do “casing” and“tubing.” In its narrow sense, a “casing” is generally considered to bea relatively large tubular conduit, usually greater than 4.5″ indiameter, that extends into a well from the surface. A. “liner” isgenerally considered to be a relatively large tubular conduit that doesnot extend from the surface of the well, and instead is supported withinan existing casing or another liner. In essence, it is a “casing” thatdoes not extend from the surface. “Tubing” refers to a smaller tubularconduit, usually less than 4.5″ in diameter. The novel stop collars,however, are not limited in their application to liners as that term maybe understood in its narrow sense. They may be used to advantage inliners, casings, tubing, and other tubular conduits or “tubulars” as arecommonly employed in oil and gas wells.

The novel stop collars also have been described in the context ofcentralizing assemblies used in cementing operations. They may be usedin centralizing assemblies for other applications, such as“casing-while-drilling” operations where a bit is attached at the end ofa liner to drill a bore and install a liner at the same time. They maybe used to limit the travel of centralizers, but they also may beincorporated into centralizers which are fixedly mounted on a liner. Forexample, a pair of novel stop collars may be joined by bow springs toprovide a centralizer that may be mounted securely in a particularlocation along a liner. A more elongated sleeve may be provided withspiral blades, and each end configured generally as is the inner end offemale sleeve 35. A male sleeve 34 then may be assembled to each end ofthe spiral-blade sleeve to mount it in a fixed location on a liner.

Moreover, while they are particularly useful in centralizing assemblies,the novel stop collars have other applications. They may be used in orwith other tools, such as cement baskets and scrapers. They also may beused as guides or protectors for sucker rods and tubing. In general,they may be adapted and employed for many different applications callingfor a fixedly mounted collar around a tubular member.

While this invention has been disclosed and discussed primarily in termsof specific embodiments thereof, it is not intended to be limitedthereto. Other modifications and embodiments will be apparent to theworker in the art.

What is claimed is:
 1. A collar for mounting on a tubular member, saidmountable collar comprising: (a) a first sleeve and a second sleeve,said sleeves: i) being adapted to extend around the outer circumferenceof said tubular member; ii) being engaged by threaded connections; andiii) providing opposing inner thrust surfaces; (b) a radially expandablegripping member disposed between said opposing thrust surfaces; (c)wherein said sleeves may be drawn together by rotation about saidthreaded connection to cause said thrust surfaces to bear on saidgripping member and expand said gripping member radially inward intofrictional engagement with said tubular member.
 2. The collar of claim1, wherein said gripping member engages said tubular member withoutpenetrating said member.
 3. The collar of claim 1, wherein said grippingmember engages said tubular member without creating electrolytic contactbetween said collar and tubular member.
 4. The collar of claim 1,wherein said first sleeve is provided with external threads engaginginternal threads on said second sleeve and said gripping member isdisposed between the inner end of said first sleeve and an annularshoulder in said second sleeve.
 5. The collar of claim 1, wherein saidsecond sleeve provides radial support for said gripping member.
 6. Thecollar of claim 1, wherein said gripping member is carried in anexpanded diameter portion of said second sleeve
 7. The collar of claim4, wherein said gripping member is a compressible band.
 8. The collar ofclaim 5, wherein said gripping member is a compressible band.
 9. Thecollar of claim 7, wherein said compressible band is fabricated from anelastomer.
 10. The collar of claim 8, wherein said compressible band isfabricated from an elastomer.
 11. The collar of claim 1, wherein saidcollar comprises means for restricting loosening of said threadedengagement between said sleeves.
 12. The collar of claim 11, whereinsaid first sleeve comprises a pawl and said second sleeve comprisesdetents angularly spaced along the inner end of said second sleeve, saidpawl selectively engaging said detents to restrict loosening of saidthreaded engagement between said sleeves.
 13. The collar of claim 11,wherein a set screw is provided to restrict loosening of said threadedengagement between said sleeves.
 14. A tool for mounting on a tubular;said tool comprising a collar of claim
 1. 15. The tool of claim 14,wherein said tool is a stop collar.
 16. A centralizing assembly for atubular member, said assembly comprising: (a) a centralizer toolcomprising spacing members, said spacing members extending radiallyoutward beyond said tubular member and defining lateral channels; and(b) a stop collar of claim
 1. 17. A tubular assembly, said tubularassembly comprising a tubular and a centralizer assembly of claim 16.18. A collar for mounting on a tubular member, said mountable collarcomprising: (a) a first member and a second member, said members being:i) adapted for lateral displacement relative to each other; and ii)providing opposing inner thrust surfaces; and (b) a radially expandablegripping member disposed between said opposing thrust surfaces; (c)wherein said members may be drawn laterally together to cause saidthrust surfaces to bear on said gripping member and expand said grippingmember radially inward into frictional engagement with said tubularmember.
 19. The collar of claim 18, wherein said gripping member is acompressible band.
 20. The collar of claim 19, wherein said compressibleband is fabricated from an elastomer.